Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your request went out to the network and something went wrong out there. This refusal covers a family of network problems. The address may be malformed, so the engine cannot even find a host to dial. The host may have answered nothing, with the operating system saying connection refused. The system may have refused to listen on a port. The other side may have dropped the connection halfway. Or the answer may have been larger than the thirty two megabyte body limit. In all these cases, the engine does not hand you a fake status code or an empty body. Why? Because a failure that flows on as a value would explode three calls later, inside some innocent function that never did anything wrong. Instead, you hear about it right here. One important distinction: a status like four oh four is not a failure. It is a real answer from a real server, and your script can read it. The message shows the call and what the network said. Check the address, the port, and whether the server is running.
Text from the Rush engine's error catalogue.